1. Field of Industrial Utilization
This invention relates to a solid-electrolyte secondary cell featuring use of a copper ion conductive solid electrolyte.
2. Prior Art
With its component elements being all made of solid materials, a solid-electrolyte secondary cell is proof against liquid leakage and can adapt itself to any size of container. Further, as it can be easily reduced in size and thickness, this type of secondary cell has the advantage that it can be integrated in the same package with other electronic parts such as IC's (integrated circuits), resistors, capacitors, etc.
For making such a solid-electrolyte secondary cell, there is required a literally solid electrolyte, in place of liquid electrolyte in ordinary cells.
The RbCl-CuCl-CuI copper ion conductive solid electrolyte discovered by Takahashi, et al. in 1979 [Journal of Electrochemical Society, Vol. 126, pp. 1654, (1979)] has a high ionic conductivity on the order of 10.sup.-2 S/cm, which is well comparable to that of liquid electrolytes, and many studies have been made for the development of solid-electrolyte cells using said type of solid electrolyte.
For composing such a cell, there are required, in addition to solid electrolyte, a pair of reversible copper electrodes which can electrochemically exchange Cu.sup.+ ions with the solid electrolyte. The decomposition potential of said copper ion conductive solid electrolyte is usually 0.6-0.7 volts, so that it is necessary to use an electrode material having an electron and ion conductive network which enables dissolution and deposition or intercalation and deintercalation of Cu.sup.+ ions at a voltage below said level and which also allows smooth transfer of Cu.sup.+ ions and electrons in the electrode without causing any chemical reaction with the solid electrolyte.
Disulphides of transition metals proposed by Whittingham in U.S. Pat. No. 4,009,052 have been highlighted as an electrode material meeting said requirements, and further studies have been made on this material. There have been proposed TiS.sub.2 [Japanese Patent Application Kokai (Laid-Open) No. 201267/83] and NbS.sub.2 [Japanese Patent Application Kokai (Laid-Open) No. 263052/86] as the disulphides which can constitute a secondary cell in combination with a copper ion conductive solid electrolyte. The secondary cell has TiS.sub.2 or NbS.sub.2 for positive electrode and a mixture of Cu.sub.2 S and Cu proposed in Japanese Patent Publication No. 013709/84 for negative electrode.
These disulphides usually have a layered crystal structure and allow interlayer transfer of Cu.sup.+ ions in charging and discharging of the cell, but the region of composition where the reversible transfer of Cu.sup.+ ions is possible while maintaining the layered crystal structure is very limited. That is, when TiS.sub.2 is represented as Cu.sub.n TiS.sub.2 and NbS.sub.2 as Cu.sub.m NbS.sub.2, the range of n or m where the reversibility can be maintained is 0 to 0.2 at most. When charging or discharging is made exceeding this range, the layered crystal structure is broken to disenable the reversible transfer of Cu.sup.+ ions, making the cell unable to perform its normal function. Therefore, the conventional secondary cells of this type could not possess a large capacity, and also charging and discharging of these cells must be controlled so that they would not be charged or discharged over the limits of reversibility, and especially such control was required that they would not be brought into a zero voltage or shortcircuited state or the potential of positive electrode would not become lower than that of negative electrode.
The cell voltage is not flat but lowers uniformly in the course of cell discharging, reflecting the continuous change of activity of Cu.sup.+ ions in one crystal phase.
Further, such disulphides are thermally unstable, and especially in the case of TiS.sub.2, sulphur starts to vaporize, though slight in amount, from TiS.sub.2 at 30.degree.-40.degree. C. Thus liberated sulfur not only acts detrimentally to the cell performance at high temperatures but also becomes a cause of errors of electronic parts when the cells using such disulphides are housed integrally with electronic parts such as IC's, resistors, capacitors, etc., in the same package.